Virtually all spacecraft today are solar powered. The solar cells used are typically triple junction solar cells based on III-V semiconductor with an efficiency of 30% under the AM0 spectrum in space. These cells are mounted on rigid carbon fiber/A1 honeycomb core sandwich panels with the help of silicone adhesives. During operation in space, these solar panels are exposed to severe temperature variations, ranging from −180° C. to +100° C., for example, during an eclipse phase in a geostationary orbit. Since the thermal expansion coefficient of the structure is virtually zero, in contrast to the non-negligible expansion coefficient of the cell, severe thermal stresses are induced in the cell. While the solar panel design can be tailored in such a way that the integrity of the cells is ensured even under these thermal stresses, this is not the case if pre-existing mechanical defects are present. The externally applied stresses are concentrated at the tip of a cell crack which results in crack growth. This can result in power loss in orbit. For this reason, it is of vital importance to detect and document mechanical cell defects on a solar panel accurately.
The de-facto industry standard is a cell inspection with the help of electroluminescence. By forward biasing the entire cell, each junction emits a characteristic electroluminescence, which can be recorded by appropriate cameras. Mechanical cell defects can be clearly identified in the image. While this method is fairly reliable in detecting mechanical defects, a drawback is that each cell has to be contacted electrically, making this method quite complex to implement on an actual solar array panel. There, several cells are connected in series to form a string. Several strings are then connected in parallel. Typically, each string has to be contacted individually to a power source. In an automated inspection flow, the image acquisition camera moves along a certain path and each string is forward biased depending on the camera position on the panel. Therefore, next to the electrical contacting effort, the entire electrical layout with the start and end point of each string has to be programmed first. Another drawback of the electroluminescence inspection is that it can only be performed on fully processed cells, but not on earlier stages in the cell production.